As computer hardware and software technology continues to progress, the need for larger and faster mass storage devices for storing computer software and data continues to increase. Electronic databases and computer applications such as multimedia applications require large amounts of disk storage space.
To meet these ever increasing demands, the hard disk drive (HDD) continues to evolve and advance. Some of the early disk drives had a maximum storage capacity of five megabytes and used fourteen inch platters, whereas today's HDDs are commonly over one gigabyte and use 3.5 inch platters. Advances in the amount of data stored per unit of area, or areal density, have dramatically accelerated. For example, in the 1980's, areal density increased about thirty percent per year while in the 1990's annual areal density increases have been around sixty percent. Areal density may be increased by increasing the rate at which data may be stored and retrieved. The cost per megabyte of an HDD is inversely related to its areal density.
In general, mass storage devices and systems, such as HDDs, include a magnetic storage media, such as rotating disks or platters, a spindle motor, read/write heads, an actuator, a pre-amplifier, a read channel, a write channel, a servo controller, a memory, and control circuitry to control the operation of the HDD and to properly interface the HDD to a host or system bus. The read channel, write channel, servo controller, and memory may all be implemented as one integrated circuit that is referred to as a data channel. The control circuitry often includes a microprocessor for executing control programs or instructions during the operation of the HDD.
An HDD performs write, read, and servo operations when storing and retrieving data. A typical HDD performs a write operation by transferring data from a host interface to its control circuitry. The control circuitry then stores the data in a local dynamic random access memory (DRAM). A control circuitry processor schedules a series of events to allow the information to be transferred to the disk platters through a write channel. The read/write heads are moved to the appropriate track and sector. Finally, the HDD control circuitry transfers the data from the DRAM to the sector using the write channel. A sector generally has a fixed data storage capacity, such as 512 bytes of user data per sector. A write clock controls the timing of a write operation in the write channel. The write channel may encode the data so that the data can be more reliably retrieved later.
In a read operation, the appropriate sector to be read is located and data that has been previously written to the disk is read. A read/write head senses the changes in the magnetic flux of the disk platter and generates a corresponding analog read signal. The read channel receives the analog read signal, conditions the signal, and detects “zeros” and “ones” from the signal. The read channel conditions the signal by amplifying the read signal, to an appropriate level using an automatic gain control circuit, such as can be included in a disk-drive preamplifier. The read channel then filters the signal, to eliminate unwanted high frequency noise, equalizes the channel, detects “zeros” and “ones” from the signal, and formats the binary data for the control circuitry. The binary or digital data is then transferred from the read channel to the control circuitry and is stored in the DRAM. The processor then communicates to the host that data is ready to be transferred. A read clock controls the timing of a read operation in the read channel. The goal during a read operation is to accurately retrieve the data with the lowest bit error rate (BER) in the noisiest environment.
The goal of the automatic gain control circuit during a read operation is to generate an appropriate output signal from the associated preamplifier so that the read signal may be efficiently and accurately analyzed by the read channel. For example, if the amplitude of the output signal is too high, the gain is reduced, and if the amplitude of the output signal is too low, the gain is increased. However, as the speed and storage capacity of HDD systems improves, the variation of the resistance in the magneto-resistive (MR) read head and the variation of the MR read head signal amplitude may become wider due to the use of more sensitive circuit elements. In contrast, the input dynamic range of the read channel device may be narrower due to the use of lower voltage processing to achieve higher performance. As a result, there can be a dynamic range mismatch between the output of the preamplifier and the input of the read channel device.